1. Field of the Invention
The present invention relates to a direct modulation type voltage-controlled oscillator using a MOS varicap, and more particularly to a voltage-controlled oscillator capable of performing a linear phase modulation.
2. Description of the Background Art
As is well known, a voltage-controlled oscillator (VCO) is widely used as a device for generating a local oscillation signal of a wireless communication device or the like. In particular, a direct modulation type voltage-controlled oscillator can perform a local oscillation process and a modulation process simultaneously. The direct modulation type voltage-controlled oscillator is disclosed in US Patent Publication No. 2005/0231296A1, for example.
FIG. 8 is a diagram illustrating an exemplary configuration of a conventional direct modulation type voltage-controlled oscillator 100. The conventional direct modulation type voltage-controlled oscillator 100 comprises inductors L111 and L112, a first variable capacitor section 110 for a carrier wave component, a second variable capacitor section 120 for a modulated wave component, transistors M111 and M112, and a current source I110. The inductors L111 and L112 are connected in series with each other so as to form an inductor circuit. The transistors M111 and M112 are cross-coupled to each other so as to form a negative resistance circuit. The first and second variable capacitor sections 110 and 120 are connected in parallel with the inductor circuit so as to form a parallel resonant circuit.
The first variable capacitor section 110 includes variable capacitors VC111 and VC112, each being composed of a MOS varicap, DC cut capacitors C111 and C112, and resistances R111 and R112. The variable capacitors VC111 and VC112 are connected in series with each other. The DC cut capacitors C111 and C112 are connected, respectively, to both ends of a series-connected circuit formed by the variable capacitors VC111 and VC112. A first reference voltage Vref1 is applied to a connection point of the variable capacitor VC111 and the variable capacitor VC112 as a DC bias. A control voltage VT1 corresponding to the carrier wave component is fed to a connection point of the variable capacitor VC111 and the DC cut capacitor C111 via the resistance R111 for blocking a high frequency. Furthermore, the control voltage VT1 is also fed to a connection point of the variable capacitor VC112 and the DC cut capacitor C112 via the resistance R112 for blocking the high frequency.
The second variable capacitor section 120 includes variable capacitors VC121 and VC122, each being composed of a MOS varicap, DC cut capacitors C121 and C122, and resistances R121 and R122. The variable capacitors VC121 and VC122 are connected in series with each other. The DC cut capacitors C121 and C122 are connected, respectively, to both ends of a series-connected circuit formed by the variable capacitors VC121 and VC122. A second reference voltage Vref2 is applied to a connection point of the variable capacitor VC121 and the variable capacitor VC122 as a DC bias. A control voltage VT2 corresponding to the modulated wave component is fed to a connection point of the variable capacitor VC121 and the DC cut capacitor C121 via the resistance R121 for blocking the high frequency. Furthermore, the control voltage VT2 is also fed to a connection point of the variable capacitor VC122 and the DC cut capacitor C122 via the resistance R122 for blocking the high frequency.
FIG. 9 is a diagram illustrating an example of a unit characteristic (shown by a solid line) of a MOS varicap used in the variable capacitor. FIG. 10 is a diagram illustrating an example of a gain characteristic of a voltage-controlled oscillator using a MOS varicap. Note that in each of FIGS. 9 and 10, a dashed line indicates an exemplary characteristic of a PN varicap.
As shown in FIG. 9, in a MOS varicap characteristic, there is an advantage that a range within which a capacity value of the MOS varicap varies is larger as compared to a PN varicap characteristic. In the MOS varicap characteristic, however, there is a drawback that a range within which the capacity value of the MOS varicap linearly varies with respect to a change in a potential difference across the MOS varicap is narrower as compared to the PN varicap characteristic. Therefore, as shown in FIG. 10, in the direct modulation type voltage-controlled oscillator using a variable capacitor as the MOS varicap, there is an advantage that the gain characteristic has a larger peak gain as compared to a direct modulation type voltage-controlled oscillator using a variable capacitor as the PN varicap. In the direct modulation type voltage-controlled oscillator using a variable capacitor as the MOS varicap, however, there is a drawback that an allowable range, of the potential difference across the MOS varicap, within which the peak gain can be obtained is narrower as compared to the direct modulation type voltage-controlled oscillator using a variable capacitor as the PN varicap. Therefore, it is desirable to always use the gain characteristic obtained when the potential difference across the MOS varicap is in the vicinity of zero.
However, the potential difference across the MOS varicap fluctuates due to component variations between the MOS varicaps, a temperature change of the voltage-controlled oscillator and the like. Particularly, in the conventional direct modulation type voltage-controlled oscillator 100 described above, an influence exerted on the second variable capacitor section 120 for controlling the modulated wave component cannot to be ignored. Thus, there is a problem that it is difficult to optimally perform the modulation process by always using the peak gain.